[2,2′-Bipyridyl]-3,3′-diol in lipid vesicles: slowed down dynamics of proton transfer

Abstract

(2,2′-Bipyridyl)-3-3′-diol [BP(OH)₂] is a well-known compound that undergoes double proton transfer (DPT) that has been previously described as an intramolecular and excited state event in neat solvents and certain constrained environments. Herein, we have explored the interaction of BP(OH)₂ with small unilamellar vesicles synthesized using two kinds of lipids, dimyristoyl-L-α-phosphatidylcholine (DMPC) and dimyristoyl-L-α-phosphatidylglycerol (DMPG), which are zwitterionic and anionic, respectively. BP(OH)₂ is observed to undergo the formation of a monoketo and a diketo species in sequence along with the creation of dianions aided by the lipid membranes. Interestingly, in the present work, we could witness the formation of the dianionic species under neutral conditions in the presence of lipid vesicles. Moreover, the dynamics of the monoketo species could also be observed as the lipid vesicles supposedly slowed down the overall rate of proton transfer in BP(OH)₂. In spite of monitoring in different emitting regions of the spectrum of BP(OH)₂ in liposomes, no growth component was observed in the time-resolved fluorescence data. This indicates that in the presence of lipid vesicles all the subsequent species from the parent neutral probe develop in the ground electronic state.